Molecules containing an unpaired electron are referred to as free radicals. Free radicals are extremely reactive. Partial reduction of oxygen by mammalian biological systems produces the free radicals, superoxide and hydroxyl. The two electron reduction product of oxygen, hydrogen peroxide, is also produced but contains no unpaired electrons. However, it is usually a precursor to the hydroxyl radical which is the most reactive of the three. The hydroxyl free radical will react with almost any biomolecule. Examples of such biomolecules include nucleic acids, lipids, and proteins. The hydroxyl radical will oxidize the biomolecule by either extracting a hydrogen atom from the biomolecule or by adding directly to the biomolecule itself. This oxidation by the hydroxyl free radical transforms the biomolecule into a radical which will readily react with molecular oxygen, thereby forming what is referred to as a peroxyl free radical. The resulting peroxyl radical will react with another biomolecule producing a free radical, which will also be transformed into another peroxyl radical as described above. The initial presence of the oxygen free radical initiates a chain reaction in which a number of biomolecules in the organism are oxidized. By oxidizing lipids, these free radicals can affect cell membranes, their permeability, ion channels, cell function, etc. By oxidizing proteins, they can alter enzymes, muscular function, nerves, etc. By oxidizing nucleic acids, they can affect DNA, RNA, and their expression products.
Recent research has indicated that excessive levels of these oxygen free radicals are associated with the tissue damage which occurs in a number of disease states such as stroke, myocardial infarction, senile dementia, shock, etc. Stroke and septic shock in particular are disease states in which radical-induced tissue damage is prevalent. Recent research has also shown that spin trapping agents may be utilized to terminate the reaction cascade described above, thereby preventing or minimizing any tissue damage. Oxygen free radicals and carbon centered radicals will react more readily with the spin trapping agent than with a biomolecule. The reaction with the spin trapping agent will result in the formation of a stable radical adduct and thus will terminate the chain reaction that is typically associated with oxygen radicals. Most tissue damage results from the chain reaction that is initiated by the oxygen radical rather than by the oxygen radical itself. The mechanism of action by which oxygen radicals cause tissue damage, as well as the use of spin trapping agents to prevent this damage, is described more fully by Floyd, FASEB Journal, Vol. 4, page 2588 (1990).
Nitrones 3,4-dihydro-3,3-dimethylisoquinoline N-oxide (A) and spiro cyclohexane-1,3'!3,4-dihydroisoquinoline N-oxide (B)(FIG. 1) are cyclic analogs of the known radical scavenger PBN, which had previously been developed. Embedding the nitrone moiety in a cyclic system should give an essentially planar molecule in which good orbital overlap exists between the nitrone double bond and the aromatic ring. Molecular modelling studies indeed suggest that in the lowest energy conformation of A, the nitrone double bond is coplanar with the aromatic ring, whereas the corresponding relationship with PBN is ca. 30.degree. offset from coplanarity. These predictions have been supported by X-ray crystallography. This increased degree of conjugation, relative to PBN, was expected to make the nitrone function in the cyclic analogs more accessible to radicals, and result in more stable product radicals. Experimentally, both A and B are more potent inhibitors of lipid oxidation, and better hydroxyl radical traps, than PBN. See U.S. Pat. No. 5,397,789, issued Mar. 14, 1995, incorporated herein by reference. ##STR1##